Women who averaged three to six drinks a week had a 15% higher risk of invasive breast cancer compared with nondrinkers. The excess risk increased to 50% in women who averaged more than 30 drinks a week.

Binge drinking also increased the breast cancer hazard but the frequency of alcohol consumption did not, according to a report in the Nov. 2 issue of the Journal of the American Medical Association.

"We observed a 10% increase in risk with each 10 g per day of alcohol intake," Wendy Y. Chen, MD, of Harvard, and co-authors wrote in their assessment of their findings.

"Consistent with other studies, we did not find any difference between type of alcohol beverage," they added.

Although the mechanism for the association between alcohol and breast cancer remains unclear, one "probable explanation" relates to alcohol's tendency to increase circulating estrogen levels. The observation is consistent with studies showing a stronger influence of alcohol on hormone-dependent breast cancer, they wrote.

In general, the findings amount to "old news," in the view of clinicians and scientists who responded to a request for comment from MedPage Today and ABC News.

"[The association] has been shown over and over and has been known for many years; one of the very few modifiable risk factors for breast cancer," Karin Michels, ScD, PhD, of the Harvard School of Public Health, said via email.

Balancing alcohol's breast cancer risks and cardiovascular benefits also is old news, she added, noting that women should weigh their family history: Does it come down more on the side of cancer or heart disease?

Assessing the risks and benefits is the key issue, according to Tim Byers, MD, of the Colorado School of Public Health in Aurora.

"For most women at average risk for both conditions, the balance favors heart disease (i.e., moderate drinking has a net benefit) as heart disease is much more common than breast cancer; but for women at high risk for breast cancer that balance shifts," said Byers.

The results "will not change how we counsel and [are] very consistent with the many prior studies that have shown the same thing," Kathy Helzlsouer, MD, of Mercy Medical Center in Baltimore, said in her email response.

In their introduction, Chen and co-authors acknowledged that numerous previous studies had examined the relationship between alcohol consumption and breast cancer, but they asserted that the studies had produced inconsistent, and therefore inconclusive, results.

Many of the earlier studies did not update information on alcohol consumption over time, the researchers noted. They also did not examine the impact of the frequency of alcohol consumption and binge drinking on breast cancer risk, the authors added.

Moreover, previous studies did not take into account factors that can alter a women's risk of breast cancer, such as first full-term pregnancy and exposure to ionizing radiation.

The limitations of prior studies led Chen and colleagues to examine the relationship between alcohol consumption and breast cancer in the Nurses' Health Study, a cohort of 121,700 female nurses enrolled and followed since 1976.

Each NHS participant completed baseline questionnaires that included items about risk factors for cancer and cardiovascular disease. Participants have received follow-up questionnaires by mail every two years since enrollment.

The authors chose 1980 as the baseline for their data analysis, as that was the first year that NHS investigators asked study participants about alcohol consumption. Follow-up continued through mid-2008, at which time 7,690 cases of invasive breast cancer had been diagnosed during 2.4 million person-years.

For the primary endpoint of invasive breast cancer, the investigators calculated relative risk on the basis of average cumulative alcohol consumption from 1980, taking into account updated information provided on follow-up questionnaires.

Chen and colleagues separated the study population into quintiles of daily alcohol consumption, ranging from 0 to ≥20 g. Using nondrinkers as the reference, the investigators observed an increasing risk of invasive breast cancer with increasing daily alcohol intake:

0.1 to 4.9 g: RR 1.06 (95% CI 0.99 to 1.12)

5.0 to 9.9 g: RR 1.15 (1.06 to 1.24)

10.0 to 19.9 g: RR 1.22 (1.13 to 1.32)

20.0 to 29.9 g: RR 1.20 (1.07 to 1.36)

≥30 g: RR 1.51 (1.36 to 1.70)

The analysis yielded a trend for a 10% increase in breast cancer risk for each 10-g increase in alcohol consumption (P<0.001).

The investigators found that the largest number of drinks consumed in a single day (binge drinking) also predicted breast cancer risk (P<0.001).

They did not find that age affected risk -- in women 40 and younger and those older than 40, alcohol consumption carried the same significant association with breast cancer risk (P<0.001 for both comparisons).

Consistent with previous studies, Chen and co-authors found that alcohol consumption had a greater impact on the risk of hormone receptor-positive breast cancer. The relative risk compared with nondrinkers ranged from 1.29 for women reporting the lowest alcohol consumption to 2.45 for those reporting the highest.

Although the findings are not particularly new, some respondents to the MedPage Today-ABC News query said the analysis added new information about the relationship between alcohol consumption and breast cancer risk.

"This is an important study," said David Jernigan, PhD, of the Johns Hopkins School of Public Health. "In terms of counseling women, what is critical to understand is that there is no evidence of a protective effect of moderate alcohol use for persons below age 40. Increasingly, the group of women at highest risk for excessive alcohol use is young and young adult women.

"In addition to the standard warnings against excessive alcohol use in this age group, there is now additional evidence showing that (particularly if you have other risk factors for breast cancer such as family history) any amount of drinking over the life course increases risk for breast cancer."

James Garbutt, MD, of the University of North Carolina in Chapel Hill, said, "What is important about this study is that it followed women over time and confirmed and extended the link between alcohol consumption and breast cancer. Given that many women drink alcohol in our society, this is an important observation and one that women should be aware of.

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